To install and use a movable basket stand on the floor surface of a gym or the like, while preventing the rear part of a goal stand base from floating up, which is caused by the floor surface being vibrated and deformed so that the backboard and the goal ring are shaked and fluctuated in height.

In an underfloor foundation which has a movable basketball goal stand 1 installed under a floor surface of a gym or the like or in a steel pipe post, an embedded pipe 11 is embedded. A mounting bed 13 which is elevatable and lowerable or attachable and detachable is threadedly mounted. The mounting bed 13 and a receiving metal fixture 8 of the mounting bed 13 of a height-adjustable leg 7 installed in a basketball stand base 2 are structurally fixed.